The Prince of Wales and Duchess of Cornwall said they had been following the case of missing Madeleine McCann "closely and with deep concern" and "fervently hoped" she would be reunited with her family.
The couple's remarks were made in a statement from Clarence House and were the latest in a series of high profile messages of support for the family.
It was revealed earlier that Prime Minister-in-waiting Gordon Brown had spoken personally to the father of missing Madeleine to pledge support from the top of the Government for the family's efforts to find her.
Entrepreneur Sir Richard Branson, footballer Wayne Rooney and Harry Potter creator JK Rowling, have all offered financial rewards a multi-million pound reward for her safe return.
Portuguese police were forced to make an embarrassing U-turn after Madeleine's parents were desperate to release details about a man seen carrying what was thought to be a child on the night Madeleine went missing and had threatened legal action.
But after a meeting between the couple and senior Portuguese officials, the police relented and a physical description was issued.
Gerry McCann said the talks were amicable and constructive and insisted the sighting could prove "significant and relevant" in finding the four-year-old. It is 23 days since Madeleine was snatched from her bed in the Algarve town of Praia da Luz.
Mr McCann said he and wife Kate wanted "more than anything" to get their daughter back. Appealing for help, he said: "Any information, no matter how unimportant you think it could be might be vital in helping the Portuguese and British police find our daughter."
The McCanns are convinced that a white man aged around 35 to 40 spotted by a friend of the couple near their apartment at 9.30 pm was holding Madeleine. He was medium build, 5ft 10in tall, wearing a dark jacket and beige trousers.
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Portuguese police have known about the sighting for three weeks but because of laws which forbid the release of details of an investigation the sighting could not be publicly released.
